I wouldn't worry about 210 sitting in 90 temps. As long as you have at least a 4lb pressure cap on it and the radiator isn't boiling you will be fine. You can always bring it up off idle and that will usually cool it down some also. My stuff will get in the 210 range or even a bit more in summer and I have never had a problem. If you do shut it off and restart it to shut it off again will more than likely cause it to puke out antifreeze and make things worse.....
